Glass-ceramic articles have many applications, e.g., as refractory kitchen dishes resistant to thermal shock and especially also as cooking surfaces for kitchen ranges. These articles are generally provided with decorations (e.g., colored designs), whether it be for purely optical reasons or, e.g., to delineate the cooking locations on cooking surfaces. Ceramic colors based on porcelain enamel are used as colors for the decorations.
Ceramic colors lay on the surface of the article. They generally comprise a binder, typically a glass of suitable melting point, and coloring pigments, typically oxides. The glass used as the binder can also be colored.
The ceramic colors generally are in the form of a powder which is dispersed in a disperant to form a paste. Ceramic colors are applied according to conventional techniques such as, e.g., screen printing or by the transfer picture technique (decals), and are then baked-in.
As in known, glass-ceramics are produced from a ceramizable glass by a thermal treatment according to a certain temperature-time program (ceramization). Temperatures up to about 1,000.degree. C. are reached during such a program.
For reasons relating to production, and conservation of energy, baking of the decorative colors is preferably performed at the same time as ceramization, although the ceramization temperatures are unusually high compared with the usual baking temperatures.
As in known, there are numerous glass-ceramics. These glass-ceramics can have quite different chemical compositions and different conversion mechanisms. .It is evident to one skilled in the art that, due to the high baking temperature during the ceramization, inter alia, for many colors defects or problems can occur.
These include defects in the decorative color applied to the glass-ceramic, defects to the ceramizable glass in the form of undesired shades, insufficient adhesiveness or thermal cycling stability of the decoration, excessive roughness and the resultant problems of cleaning associate therewith, insufficient stability toward alkalis (cleansing agents) or toward acids (e.g., citric acid), as well as running, unsharp shapes (halo formation) of the decoration. Several of such defects can occur simultaneously.